Nuclear magnetic resonance (NMR) spectroscopy provides a quantitative, non-destructive measure of chemical concentrations in complex mixtures both in vitro and in vivo. In many samples, the spectral lines are narrow while the range of resonance frequencies is broad, and there is little overlap of spectral lines.
Nuclear magnetic resonance spectra of complex chemical mixtures often contain, however, unresolved and unseen spectral components, especially when a strong background spectrum overlaps weaker peaks. For example, mixtures of biomolecules, such as blood, urine, and brain tissue, often contain a large number of compounds with spectral lines mostly overlapped by a few dominant metabolites. Furthermore, some abundant metabolites, such as glutamine and glutamate, have such similar structures that their spectra are nearly identical and are difficult to resolve from one another. In all these cases, no amount of signal averaging can improve the resolution.
While improved resolution can be achieved using higher-field instruments, this is often a costly solution. An alternative is to use quantum filters to remove undesired spectral components and to select those of interest. These techniques work by creating a quantum coherence on a target molecule and then applying phase cycling or gradient filters to selectively suppress either the target or the background signals. Common applications include water and fat suppression as well as metabolite-specific enhancement in magnetic resonance amino-acid-specific enhancement in the spectroscopy of proteins, and metabolic analysis of blood and urine. However, quantum filters have had limited success in differentiating molecules with very similar structures, such as glutamine and glutamate.